Monomers of the protein actin polymerize to form long, thin fibers. These are about 8 nm in diameter and, being the thinnest of the cytoskeletal filaments, are also called microfilaments. (In skeletal muscle fibers they are called "thin" filaments.)
Some functions of actin filaments:

These cytoplasmic fibers average 10 nm in diameter (and thus are "intermediate" in size between actin filaments (8 nm) and microtubules (25 nm)(as well as of the thick filaments of skeletal muscle fibers).

There are several types of intermediate filament, each constructed from one or more proteins characteristic of it.

keratins are found in epithelial cells and also form hair and nails;

nuclear lamins form a meshwork that stabilizes the inner membrane of the nuclear envelope;

neurofilaments strengthen the long axons of neurons;

vimentins provide mechanical strength to muscle (and other) cells.

Despite their chemical diversity, intermediate filaments play similar roles in the cell: providing a supporting framework within the cell. For example, the nucleus in epithelial cells is held within the cell by a basketlike network of intermediate filaments made of keratins. (photo at right)

In the photo (courtesy of Mary Osborn), a fluorescent stain has been used to show the intermediate filaments of keratin in two epithelial cells. Note the basketlike arrangement of filaments around each nucleus.

Different kinds of epithelia use different keratins to build their intermediate filaments.
Over 20 different kinds of keratins have been found, although each kind of epithelial cell may use no more than 2 of them. Up to 85% of the dry weight of squamous epithelial cells can consist of keratins.

kinesins (most of these move toward the plus end of the microtubules) and

dyneins (which move toward the minus end).

Some examples:

The rapid transport of organelles, like vesicles and mitochondria, along the axons of neurons takes place along microtubules with their plus ends pointed toward the end of the axon. The motors are kinesins.

The migration of chromosomes in mitosis and meiosis takes place on microtubules that make up the spindle fibers. Both kinesins and dyneins are used as motors [Link].

Vincristine, a drug found in the Madagascar periwinkle (a wildflower), binds to tubulin dimers preventing the assembly of microtubules. This halts cells in metaphase of mitosis.

Taxol®, a drug found in the bark of the Pacific yew, prevents depolymerization of the microtubules of the spindle fiber. This, in turn, stops chromosome movement, and thus prevents the completion of mitosis.

Because the hallmark of cancer cells is uncontrolled mitosis, both vincristine and Taxol are used as anticancer drugs

Cilia and Flagella

Cilia and flagella are built from arrays of microtubules. They are discussed on a separate page. Link to it.